1. Field of the Invention
The invention relates to a control system for an automobile engine, and more particularly, to an engine control system including a combustion control means for selecting combustion modes of an engine between a stratified charge combustion mode and a homogeneous charge combustion mode according to engine operating conditions and a fuel control means for enforcing a fuel cut upon an occurrence of a specified engine operating condition.
2. Description of the Related Art
Typically, in order for a fuel injection control system of the type injecting fuel directly into a combustion chamber of an internal combustion engine, and firing the fuel to reduce impact or shock imposed on the driver, a control means is equipped to prevent a change from an intake stroke injection mode to a compression stroke injection mode and vice versa. As described in, for example, Japanese Unexamined Patent Publication No. 7-279729, such a fuel injection control system switches control from an intake stroke injection mode to a compression stroke injection mode and vice versa according to engine operating conditions, decides fuel cut patterns according to engine operating conditions and practices a fuel cut in the fuel cut pattern, and holds either one of the fuel injection modes in the event where a specified engine operating condition is effected after resumption of fuel injection.
The fuel injection control system described in the above publication has a problem that the engine is apt to cause a miss fire due to a lack of intake air introduced into the combustion chamber if holding the compression stroke injection mode. This is because the fuel injection control system is adapted to hold either one of the fuel injection mode, namely the intake stroke injection mode or the compression stroke injection mode, when the conditions of interruption of the fuel cut is met as a result of a decline in engine speed after the fuel cut control has once begun. Specifically, in order for, in particular, a lean burn engine which burns a lean fuel mixture of air-fuel ratios between 40 and 140 in a stratified charge combustion mode to prevent the catalyst of an exhaust emission purifying device from encountering a decline in temperature due to a low temperature of intake air during the fuel cut control, the fuel injection control system controls an intake air regulator, such as an electrically controlled engine throttle or an idle speed control valve, to remain closed. The engine generally runs with a low engine load when the fuel injection control system resumes fuel injection after a fuel cut. At this time, while it is preferred for the engine to practice stratified charge burning in the compression stroke injection mode, nevertheless, a delay in supplying air is unavoidable even if the fuel injection control system opens the intake air regulator simultaneously with resuming fuel injection, as a result of which, a delay in stratification of a fuel mixture is encountered due to a lack of intake air, then the fuel mixture is partly enriched in excess around the ignition plug and miss fired.